Thing is the difference between a Fission and Fusion reactor design is such that you will need to rebuild tge Fusion reactor to get the PU basicly biyearly.
While with a Fission design you can just shut down, pop open a hatch and replace the rods, the crank it back up within a few days.
You're going to need to change out the lithium every now and then anyway, and I don't see any obvious reason it would be
difficult to make it swappable in the same way as fuel rods.
AFAIR, U-238 does not fission directly - it needs to turn into Pu-239 first. But unless it's done in extremely controlled circumstances the Pu-239 risks being fissioned and wasted.
There's a crapload of very special requirements to get a successful reactor that does anything, as some Uranium isotopes are "poison" to the fission process (will eat neutrons without doing anything productive with them).
All actinides can fission from high-energy neutrons. U-238 doesn't fission from
thermal (low-energy) neutrons; it
can fission from the higher-energy fission neutrons (>1 MeV), but it usually doesn't and therefore won't support a chain reaction.
The "fusion neutrons" from D-T fusion are
very-high-energy - 14 MeV - as the vast majority of the fusion energy is carried by the neutron. They will fission U-238 with no problems. This is, in fact, used in most hydrogen bombs; the radiation case and in some cases the secondary's tamper are made of depleted uranium (U-238), which won't support a chain reaction (and thus has no criticality issues) but
will fission from the fusion neutrons and therefore add more bang. Speaking of which, the reason Castle Bravo was 3x bigger than expected and killed people is because the fusion neutrons fissioned the lithium-7 as well as the lithium-6 in the secondary - lithium-7 fission actually
consumes energy rather than releasing it, which is why they thought it wouldn't happen, but fusion neutrons have such immense energy that they split it anyway.
The only uranium isotope that's not either fissile or directly fertile is U-236, and then only in a thermal-neutron mode (Np-237 will support a chain reaction with fast neutrons, though not with thermal neutrons). For the most part, the so-called "neutron poisons" that cause issues are fission
products that absorb neutrons. And even then, more dakka works; U-235 and Pu-239 are fissile enough that they can maintain a chain reaction despite some amount of poison (control rods are literally neutron poison
deliberately placed inside the reactor core).
